When you view a lead, the system acquires a lock on that lead for you, so the lead is removed from all queues, and other agents won't be able to call it. When you perform an action on the lead, the lock is released. If you do nothing, the lock will expire and the lead will be reinserted into the queues after about an hour.

If you only want to inspect leads without calling them and changing their state, then you can set "View leads only" from your preferences. This will allow supervisors and admins to view leads without setting the lock, so agents can work their queues uninterrupted.